It has become progressively more usual in present-day telecommunication systems to transmit different types of information on a common transmission line in different frequency bands. One example of such a system is ADSL (Asymmetric Digital Subscriber Line) which enables standard telephony in speech form (POTS=Plain Old Telephone Service) and broadband services, such as video or data, to be transmitted in parallel over a conventional copper pair cable network. This technique allows the existing copper pair cable network to be utilised more effectively, which is of the greatest interest in view that it is very costly and time consuming to replace the copper cable with high capacity optical fibres, for instance. There is also a high-speed variant of ADSL called VADSL or VDSL (Very High Speed ADSL).
Because POTS traffic only uses the frequency band up to 4 kHz, a large part of the available frequency band in a transmission line often goes unused. In ADSL systems, a higher frequency range of, e.g., 25-1000 kHz is used for broadband services of about 1.5-6 Mbit/s. The information transmitted in the higher frequency range is called ADSL traffic or ADSL signals. When the telecommunication traffic arrives at receiver of a subscriber or of an exchange of an operator, the POTS traffic and ADSL traffic are separated from one another, often with the aid of so-called splitter filters. The splitter filter includes a low-pass filter between the transmission line and the subscriber telephone/the line interface board of the telephone exchange and a high-pass filter between the transmission line and an ADSL modem belonging to the subscriber or installed in the telecommunications exchange.
In order to obtain optimal transmission properties and to avoid echo, it is desirable that the nominal impedance of the splitter filter be similar to the transmission line impedance or to the impedance resulting from a combination of the transmission line impedance and a terminating impedance in the remote end of the transmission line. The impedance of a transmission line is complex, whereas the terminating impedance may be either complex or resistive. The expression “transmission line impedance” used hereinafter includes both cases where precisely the impedance of the transmission line is meant and when the combination of the transmission tine impedance and the terminating impedance is meant.
It is particularly important to avoid echo in the speech band and consequently of particular importance that the impedance of the low-pass filter is matched satisfactorily to the transmission line impedance. Because the transmission line impedance is complex, it is desirable that the impedance of the low-pass filter is also complex. It is also desirable that the filter is passive so as to avoid power supply problems and to ensure that the filter will continue to function in the event of a power failure. However, no method of providing a passive low-pass filter with an impedance that is complex in a manner, which will allow the filter impedance to be matched satisfactorily to the complex impedance of a transmission line, has been known hitherto.
One solution to this problem involves embedding a passive low-pass filter that has a resistive impedance between General Impedance Converters (GIC). The GICs transform the resistive impedance of the filter so that the arrangement of GICs and filter has a complex impedance, seen from the transmission line. The GICs used in this connection include transformers and amplifiers and are thus active. Thus, the whole of the filter arrangement including low-pass filters and GICs is active and need for a power supply is still unavoidable with this method.
The technique of embedding a filter having a resistive impedance between two GICs in order to obtain a complex nominal impedance is known, e.g., from U.S. Pat. No. 5,623,543 and the article “ADSL and VADSL Splitter Design and Telephony Performance” by John Cook and Phil Sheppard, published in IEEE Journal on Selected Areas in Communications, Vol. 13, No. 9, December 1995. The importance of impedance matching is also discussed in the aforesaid patent specification and in said article.
Passive filters for systems in which different types of information are transmitted over one and the same transmission line in different frequency bands are described in U.S. Pat. No. 5,848,150 and U.S. Pat. No. 4,764,922. However, none of these documents addresses the problem of impedance matching to the complex impedance of a transmission line.